In recent years, increasing demand has arisen for improvement of the fuel consumption of engines for automobiles. Since an important point for fuel consumption improvement is the attachment accuracy of a spark plug to an engine, demand has arisen for improvement of the attachment accuracy. Requirements for attachment accuracy improvement include, for example, the amount of protrusion of a spark plug into a combustion chamber, and the direction of an outer electrode (ground electrode) of a spark plug.
Hitherto, a gasket having a hollow structure has been replaced by a gasket formed of an annular plate material for the purpose of improving the attachment accuracy of a spark plug. A known technique regarding a gasket formed of a plate material is disclosed in, for example, Patent Document 1. However, this technique is insufficiently devised in terms of securement of gas-tightness and suppression of loosening.